DE3730829C2 - Overload protection for the excitation circuit of a synchronous machine - Google Patents

Description

The invention relates to overload protection for the excitation circuit a synchronous machine evaluating measured variables of the stator circuit (stator current, stator voltage) Machine (DE 31 35 919 A1).

In the overload protection devices known from practice the excitation current is either via a shunt or equal current transformers detected as direct current or in a three-phase Excitation circuit feed determined via current transformers. The way The measured value obtained is a thermal image of the exciter based on the circle that is exceeded when the permissible Warming gives a warning or trigger signal.

The detection of the direct current sets one for the measurement DC current accessible through shunt or DC converter Excitation circuit ahead, what with machines with rotating diodes does not apply. Measuring the three-phase current requires that this is exactly proportional to the excitation direct current, which is not guaranteed in all malfunctions.

From DE 31 35 919 A1 is a short-circuit diode protection known for a brushless alternator, who has an excitable field winding. The order comprises a circuit unit for sensing the voltage difference ferentials on the excitation field and to combine the voltage differentials with a generator load current. So that becomes a Output signal generated within a predeterminable Range of values over a normal generator load current range falls off.

DE-AS 12 38 092 relates to an arrangement for a syn chrongenerator, starting from at the terminals of Ge nerators existing measured variables determined an operating state  becomes. If a limit value is exceeded, a countermeasure is taken took hold of the voltage regulator of the synchronous generator. It is a special control arrangement that for the specific operational case of falling out of step (also Called under-excitation).

The invention has for its object without measuring Sizes of the excitation circuit in a simpler way a flawless Free overload protection for all versions of the excitation circuit of synchronous machines to create the only stand sizes used, which is also required for the other protective devices will.  

The task is solved by the Features of claim 1.

The invention is based on the known voltage diagram a synchronous machine:

Fig. 1 shows the general voltage diagram with different longitudinal reactances (X _d ) and transverse reactances (X _q ).

The relationship can be inferred from this

and the knowledge that I _e ∼ E can be set (3)

Mean in it
U = stator voltage
I = stator current
E = magnet wheel voltage
X _d = synchronous longitudinal reactance
X _q = synchronous cross-reactance
ϕ = phase angle
ϕ _N = nominal phase angle
ϑ = magnet wheel angle
I _e = excitation current

The sizes X _d and X _q are known for every machine. The sizes U and I are easy to measure values. The magnet wheel angle ϑ can easily be determined by measuring the angle between U and U + IX _d according to equation (2). Thus, the magnet wheel voltage can be determined according to equation (1) and the excitation current according to the proportionality according to equation (3).

For the task at hand here it is sufficient in most cases to use the simplified voltage diagram according to FIG. 2. This results from Fig. 1 when X _d = X _q .

This condition is very important for synchronous machines with a turbo rotor well fulfilled. For synchronous machines with stub poles this condition for the application to be considered here also well fulfilled since

• 1. to take into account the influence of saturation in the over excited working range of the synchronous machines for X _d the saturated longitudinal reactance X _dges must be used and
• 2. The following may be set in the phase angle range between ϕ _N and ϕ = 90 °: X _dges ≈ X _q .

This simplifies the relationship (1)

This relationship (4) can easily be simulated by a circuit according to FIG. 3.

Via a current transformer 1 arranged in the circuit of the synchronous generator G, the stator current I is obtained via an intermediate transformer 2 , from which in the arrangement 3 via an accordingly the machine constant X _q adjustable resistance and a 90 ° angle of rotation of the inductive voltage drop I · X _q is formed. The stator voltage U is obtained from a voltage converter 4 via the intermediate converter 5 and vectorially added to the voltage drop I · X _q . This geometric sum E = U + I · X _q is fed via an intermediate converter 6 to a thermal image 7 of the excitation circuit. This activates a downstream relay 8 according to a current-time function that is usual for a thermal image (e.g. according to DIN IEC 255 / VDE 0435 part 3011, 1984), which uses its contact to initiate a hazard warning or generator shutdown if the permissible limit heating is exceeded.

In order to ensure a physical, even with asymmetrical operating states Correct measurement can be obtained by known rotating field according to the calculation rules for symmetrical components With components of current and voltage of the three-phase system won and then continue in the manner described above be working.

In FIG. 4 is shown in FIG. 2 derived machine diagram of the synchronous machine shown, from which the maximum permissible operating area is apparent. If the limit curve a): E <∼ I _e <is exceeded, the machine is overexcited.

The other three limit curves according to FIG. 3 are:

• b) I <; this limit curve is caused by a stator overload protection captured;
• c) (-I · U · cosϕ) <; this limit curve is marked by a Reverse power protection recorded;
• d) ∡ (E, U) <; this limit curve is protected by under excitation detected.

By combining the measurements from a) to d) it is possible, only with stand sizes I and U protection for to get the measurement of the complete machine diagram.

Claims (6)

1. Overload protection for the excitation circuit of a synchronous machine, in which the excitation current I _e as a proportional variable of the magnet wheel voltage E from the measured values stator voltage U and stator current I as well as the machine parameters of the longitudinal reactance X _d and the transverse reactance X _q according to the equation is determined
where the magnet wheel angle wird is obtained from an angle measurement between U and U + IX _q and the quantity E ∼ I _e thus activated activates the overload protection when a permissible maximum value is exceeded. 2. Overload protection according to claim 1, in which the currently detected magnet wheel voltage E ∼ I _{e is} a thermal image of the excitation winding for detecting the rotor temperature. 3. Overload protection according to claim 1 and 2, in which the thermal image takes the ambient temperature into account visible electrical additional quantity is supplied. 4. Overload protection according to claim 1 to 3, in which, taking into account the saturation in the overexcited area of the synchronous machine with the saturated longitudinal reactance X _d saturated ≈ X _q, the equation E = U + IS _q applies to the determination of E ∼ I _e . 5. Overload protection according to at least one of the preceding claims 1 to 4, in which the individual limit curves of the working diagram of the synchronous machine for the current I, the magnet wheel angle ϑ, the power -I · U cosϕ and the excitation current I _e ∼ E measured or in a protective device . is determined and thus the complete machine diagram is monitored. 6. Overload protection according to claim 1 to 5, in which the corresponding Mitkompo as current and voltage values components of the three-phase system are used.